1. Field of the Invention
In the past, when petroleum was relatively inexpensive, the major portion of industrial ethanol was produced by the catalytic conversion of ethylene. There has been considerable recent interest in the production of industrial ethanol by fermentation as for gasohol production. Prior to that time, any of the developments in alcohol fermentation were made in the beverage alcohol industry, in which concern for maintaining the integrity of desirable flavor components took priority over the optimization of ethanol productivity and yields.
2. Description of the Prior Art
Systems for generating yeast in vessels independently from a main fermentation vessel or chamber has been previously described in U.S. Pat. No. 3,591,454, K. Rosen, Process Biochemistry, May, 1978, pp 25, 26 and D. Rose, Process Biochemistry, March, 1974, pg 10, for example. U.S. Pat. No. 4,081,367 describes a system for purifying carbohydrate containing waste water by aerobic fermentation with yeast. Air from the tank surface is drawn by a venturi injector into a recycle stream which is injected tangential to the parabolic shaped tank bottom to cause lenticular flow up the tank wall. The waste water is added to the top of the liquid surface. Aeration is limited by horsepower in the inefficient system, and a constantly homogeneous system is not achieved. Thus the fermenation capacity of the system is not achieved.
In copending application Ser. No. 159,953 filed June 16, 1980 and now abandoned, is described an improved system and process for continuously fermenting a carbohydrate substrate (e.g. a fermentable sugar) by inoculation with a microorganism (e.g. yeast) and by use of a small aerobic fermentation vessel and a large main anaerobic fermentation vessel. The substrate feed stream is split between the two vessels. New yeast cells are grown in the aerobic vessel and directed to the anaerobic vessel. A stream concentrated in yeast solids is separated from the alcohol stream removed from the anaerobic vessel and split for recycle to both the aerobic and anaerobic vessels.
The optimum generation of yeast, for example, requires solution of oxygen in the aqueous substrate containing all necessary nutrients at a rate sufficient to replace oxygen consumed by the metabolic process. The rate of yeast generation and hence the production capacity of an aerobic vessel is largely limited by the rate of oxygen solution. Heretofore, oxygen has been introduced by introducing air in fine bubble form, with and without stirring, into the aqueous substrate, and the rate of solution has been primarily a function of bubble surface area and time of bubble-substrate solution contact. Practical means to improve oxygen transfer, for example by using pure oxygen gas, have been too expensive in construction and operation for use in systems for producing industrial grade alcohol or other fermentation products from industrial waste substrates, for example.